Air Traffic Control (ATC) tasked with safe separation of aircraft may delineate procedures and directives for entry into certain airspace. An operator requesting permission to enter the airspace must be prepared to comply with the delineated procedures. One such procedure may include a Required Time of Arrival (RTA) defined as a time an aircraft must arrive at a specific fixed position to comply with an issued RTA clearance and remain clear of other traffic.
In the United States, the Federal Aviation Administration (FAA) may adopt a Minimum Aviation System Performance Standards (MASPS) set by the Radio Technical Commission for Aeronautics (RTCA). RTCA may periodically issue changes to its MASPS Required Navigation Performance (RNP) standards for Area Navigation. Included in a recent RTCA change, a 10 second RTA compliance window in the descent phase of flight has been set as a standard within which operators may be required to comply.
Four Dimensional (4-D) (3-dimensional position plus time) trajectory architecture may be a critical component for both FAA's Next Generation Air Transportation Modernization (NextGen) program and Single European Sky ATM Research (SESAR) program. Under this 4-D trajectory concept, Flight Management Systems (FMS) not only need to generate the 4-D trajectory of the aircraft, but also to coordinate with the flight control system to track this 4-D trajectory within required positioning and timing thresholds. This 4-D trajectory architecture requires the FMS to ensure the aircraft does indeed arrive at a metering waypoint within 30 seconds (cruise phase of flight) or within 6 seconds (descent phase of flight) of its assigned RTA.
Current FMS capabilities may easily comply with the RTA accuracy of 30 seconds at a 95% probability for metering waypoints in cruise. However, current FMSs may have significant difficulty in complying with the RTA accuracy of six seconds at the 95% probability for metering waypoints on descent due to continuously changing external variables which create a problem too difficult for the traditional FMS to solve. These variables may include 1) a much tighter time threshold requirement, 2) true airspeed may likely continuously change during descent, 3) wind velocity varies over altitude causing actual ground speed to vary, and 4) standard (among all aircraft within the airspace) descent speed profiles are required.
Optimized Profile Descents (OPD) or Continuous Descent Approaches may use idle or near-idle thrust during descents to reduce fuel consumption, engine noise and carbon emissions. With Optimized Profile Descents, aircraft may descend continuously from cruise altitude to the bottom of descent or to an initial approach fix without level path segments. However, to utilize OPDs without reducing the traffic throughput around an airport, ATC may impose a RTA at a metering waypoint to safely merge incoming traffic. If idle thrust is used during the OPD, the descent profile is a function of not only aircraft speed, aircraft weight, wind and temperature, but also aircraft platforms and engine types. Therefore, the idle descent profile may vary from one aircraft to another and from one flight to another.
One highly efficient performance characteristic of a transport category aircraft may include a constant cruise phase followed by an idle-power descent from cruise altitude to the landing. This idle-power descent may include a constant Mach descent phase, a constant calibrated airspeed (CAS) descent phase, a deceleration from constant CAS to a statutory speed (e.g., 250 knots below 10,000 feet MSL), a 250 knot descent phase, a deceleration to final approach speed, and a landing. One goal for maximum efficiency may include an idle power descent from a Top of Descent (TOD) point (often 40,000 feet mean sea level (MSL)) to 1000 feet above ground level (AGL) where engines must normally be spooled up for landing and possible missed approach. This most efficient idle-power descent, however, is often unavailable due to a variety of path constraints including additional traffic in the terminal area.
During aircraft transition from enroute to landing, air traffic controllers may frequently issue instructions (or clearances) to change aircraft trajectories based on this additional traffic. These instructions may include temporary altitude assignments, increasing or decreasing speed adjustments, and temporary off-course lateral vectoring. These instructions enable traffic controllers to manage air traffic flow while ensuring proper aircraft separation and flight safety. However, these controller instructions cause inefficiencies in performance and require aircraft to execute suboptimal maneuvers, such as stair-step descents and lengthy delays at one altitude. A stair-step descent burns significantly more fuel and generates more carbon emissions and engine noise than an uninterrupted OPD because OPDs use idle or near-idle thrust to execute a smooth speed-and-altitude profile during the descent phase of flight while complying with multiple path constraints.
To enable OPDs without reducing traffic capacity throughout the Terminal Radar Approach Control (TRACON) area, a RTA constraint is usually imposed by ATC on a metering waypoint on the boundary of the TRACON area or on an Initial Approach Fix (IAF) to enable safe merging of air traffic. Therefore, upon receiving the assigned RTA well prior to the RTA, the onboard FMS must be able to quickly construct the 4-dimensional (4-D) trajectory of an OPD in accurate compliance with the assigned RTA.
A maximum performance OPD may be the best case scenario based on fuel economy, carbon emissions, and cost to the operator. However, the maximum performance idle descent OPD may not comply with the traffic clearance requirements of the local TRACON. Consequently, a need exists for a system and related method which combines the benefits of generating a path to accurately comply with a six second RTA coupled with a maximum performance OPD to minimize fuel consumption.